When you put that line in one of the files you have mentioned, you are actually NOT modifying / overriding the original definition, but are defining a new appender in the scope of that configuration file. For that appender to be used, you need to overide / reconfigure the root logger to actually use it in the same file. You also need to copy all other configurations concerning the "File" appender, since - again - you are not modifying the original definition but creating a new one, which needs to be complete to work properly.
Lastly: Out-of-the-box, there is no support for a share-log4j.properties configuration in the extension directory. There is actually no support for overridable / customizable logging in Share at all. This is why I have implemented this feature in one of my addons.